Rotatably housed cam or cylinder locks are well known in the art. Such locks are made up of a self-contained core comprising locking pins and a fence operated by the pins. Insertion of an unlocking key aligns the pins and permits retraction of the fence thus allowing the core to be rotated and a locking cam, for example, to be disengaged. An exemplary version of the cylinder lock is made by Medeco Security Locks Inc. of Virginia and described in U.S. Pat. No. 3,722,240 to Spain et al. hereby incorporated herein by reference.
It is typically desirable to restrict access to the lock core yet permit general entry to the lock. Thus, the locking/unlocking operation and the core removing operation of the lock can be separated so that two different keys are required--a general access key to disengage the lock and a master key capable of removing the core.
In U.S. Pat. No. 4,866,964 to Hall, a removable core lock is provided with a core retaining spring clip which fits in a groove in the core and extends through a slot in a shell. When a core removal key, which is longer than a lock operating key, is inserted into the lock, the lock upon rotation lifts the spring clip to allow core removal.
On closer analysis, it can be seen that the core removal prevention mechanism described above. has a relatively low security rating because a holder of an operating key can potentially defeat the mechanism and remove the core, e.g. by lengthening the operating key. This, however, is generally not a problem in applications where access to the lock operating key is restricted to a small number of people, such as, for example, in a showcase, lock box or safe. In applications where access to an operating key is relatively widespread such as in doors of apartment, hotel or office buildings and a single master key is used for replacing the core (i.e. for changing the keying of the lock), a higher security means for preventing core removal is preferred, since anyone with an operating key can, by simple modification of the key, remove the core and determine the master key combination, and thereby obtain unauthorized access to all locks keyed to the master.
It is, therefore, desirable that both the locking/unlocking and core insertion/removal operations of the locking mechanism would be tumbler controlled utilizing different tumbler combinations. Thus, two distinct keys (operation and master) having a differing overall combination of tumbler actuating grooves could be employed and unauthorized removal of the core could be greatly inhibited.